Method and apparatus for continuously and noninvasively measuring the blood pressure of a patient

ABSTRACT

In the present invention, a method and an apparatus for measuring the blood pressure of a patient continuously and non-invasively is disclosed. The blood pressure is determined by measuring the harmonic frequencies and displacements of an arterial wall of the patient. The measurements are then calibrated against an absolute blood pressure supplied by an operator at a particular time. The blood pressure is then the sum of the relative changes of the measurements and the absolute blood pressure.

This is a continuation of application Ser. No. 07/270,224, filed Nov.14, 1988 and now U.S. Pat. No. 4,960,128.

TECHNICAL FILED

The present invention relates to a method and an apparatus forcontinuously and non-invasively measuring the blood pressure of apatient and, more particularly, the present invention relates tomeasuring the harmonic frequencies and displacement components of anarterial wall, and converting the measurements into a waveform signalemulating the signal from an invasive arterial sensor.

BACKGROUND OF THE INVENTION

Methods and apparatuses for the detection of the blood pressure of apatient are well-known in the art. One type of apparatus involves theuse of an invasive arterial line. In an arterial line device, a sensoris inserted into the artery of a patient. The signal from the arterialsensor is supplied to a patient monitor which calculates, among otherparameters, the blood pressure of a patient. The patient monitor is alsocapable of sending a signal to the sensor to determine if the sensor iselectrically connected to the patient monitor. Although such apparatusesprovide continuous and accurate blood pressure data, there are manydisadvantages. First, there is a potential risk of infection. Further,the procedure is costly and consumes valuable health care takers' time,in that it is a surgical procedure involving the insertion of a catheterinto a patient's artery. In addition, the patient experiencesdiscomfort.

In the area of non-invasive devices, one prior art teaches the automaticexpansion of pressure cuffs and the measurement of the blood pressurebased thereon. However, such devices can only be used intermittently.Prolonged and frequent use can lead to patient discomfort.

U.S. Pat. No. 4,669,485 teaches using two cuffs to measure the bloodpressure of a patient. The absolute blood pressure of the patient isinitially measured. Thereafter, the second cuff is maintained at a lowpressure, continuously, to monitor continuously the relative bloodpressure of the patient. In this manner, the blood pressure of thepatient can be continuously monitored. However, such a device stillinvolves using blood pressure cuffs, which can be a source of discomfortto the patient and the device is subject to patient motion and as aconsequence degraded results.

In column 4, lines 39-column 5, line 3 of U.S. Pat. No. 4,669,485,reference is made to yet another prior art non-invasive device whichmeasured the arterial wall displacement and used those measurements todetermine the relative blood pressure. However, the use of only thedisplacement measurement is subject to error.

Reference is also made to U.S. Pat. No. 3,318,303. In that reference,the output of an external microphone sensor is used to determine theblood pressure of a patient. In particular, the so-called "Korotkoff"sound may be determined.

U.S. Pat. No. 4,203,451 discloses that a data processor can be used toreceive signals from the Carotid transducer.

U.S Pat. No. 3,773,033 discloses the use of an arterial vibration sensorto monitor the performance of the heart and arteries during a successionof cardiac cycles. However, a pressure cuff also applies variablepressure to a specified artery being monitored by the arterial vibrationsensor.

Heretofore, none of the prior art devices or references has suggested anon-invasive apparatus for continuously and accurately measuring theblood pressure of a patient using data obtained from the arterial wall,thereby providing for greater patient comfort. Further, none of theprior art devices has checked for the integrity of the sensor, apartfrom it being merely electrically connected to the monitor.

SUMMARY OF THE INVENTION

In the present invention, an instrument for continuously andnon-invasively measuring the blood pressure of a patient is disclosed.The instrument has a non-invasive sensor for measuring continuously thefrequencies and displacements of an arterial wall and for generatingcontinuously a first signal in response thereto. The first signal isprocessed continuously to produce a continuous processed first signal.The instrument further receives a calibration signal which is indicativeof the absolute blood pressure of the patient at a particular point intime. The processed first signal corresponding to the particular pointin time is stored. Finally, the instrument compares continuously thecontinuous processed first signal to the stored processed first signaland generates continuously a signal indicative of the continuous bloodpressure of the patient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block level diagram of the apparatus of the presentinvention.

FIG. 2(a-d) are detailed schematic diagrams of various portions of theapparatus shown in FIG. 1.

FIG. 3(a-f) are schematic diagrams showing representative waveformsignals processed and generated by the apparatus of the presentinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, there is shown a block diagram of an apparatus 10of the present invention. The apparatus 10 comprises a sensor assembly12 which measures continuously, with time, the external displacement andthe harmonic frequencies of the displacement caused by the lateralexpansion of an arterial wall of a patient. The pattern of expansionsand contractions is reflective of the changes in the pressure wave atthis site in the arterial system. In one embodiment, the sensor assembly12 has a piezoelectric sensing material, such as Kynar film 14. TheKynar film 14 generates a signal which is directly coupled to a sensingamplifier 16, located within the sensor assembly 12. The sensingamplifier 16 transforms the impedance of the signal from a high value,on the order of several megohms of the sensing film 14, to a low valuewhich can be transmitted over a cable 18 (typically on the order of 10feet). The sensor assembly 12 is positioned at any site of a patientwhere the artery is close to the surface of the skin, such as over theradial artery.

The sensing film 14 can be excited by an excitation signal supplied fromthe control microprocessor 54 through the control microprocessor bus 52,through an amplifier 15 and through the cable 18 to a diode 13. Theexcitation signal causes the film 14 to mechanically transduce, i.e.flex. The excitation signal is applied momentarily (on the order of onemicrosecond), and is thereafter removed. Because the sensing film 14will have been flexed by the excitation signal, upon removal of theexcitation signal, the sensing film 14 will resonate and move in theopposite direction. This movement in the opposite direction causes theproduction of a response electrical signal which is supplied along thecable 18. The detection of this response signal indicates that thesensing film 14 is not only electrically connected and is adapted totransduce, but is also in fact electromechanically ready to transducethe arterial wall movements.

The sensor assembly 12 is then positioned to detect arterial wallmotion. The signal detected by the sensor assembly 12, caused by themotion of the arterial wall, is then supplied to an input amplifier 20which removes any DC component from the signal (FIG. 3a). The inputamplifier 20 comprises an AC voltage amplifier and high pass filter 22which receives the signal from the cable 18 and supplies an outputthereof to a programmable gain DC voltage amplifier 24. The purpose ofthe programmable gain DC voltage amplifier 24 is to compensate forvariations in the sensitivity of different sensors and physicalvariations among different patients. The output of the programmable gainDC voltage amplifier 24 is then supplied to a fixed gain DC amplifierand low pass filter 26. The DC amplifier and low pass filter 26 providesan output which is supplied to an analog-to-digital converter 28. Theoutput of the input amplifier 20 is set to optimize the signal of theanalog-to-digital converter 28.

The analog-to-digital converter 28 is a bipolar 10-bit A-to-D converterwhich receives a reference voltage 30 of 2.5 volts. Theanalog-to-digital converter 28 digitizes the input analog signal at therate of approximately 250 hertz or one conversion each fourmilliseconds.

The output of the bipolar analog-to-digital converter 28 is thensupplied to a first-in-first-out (FIFO) buffer 32. The data is stored inthe FIFO buffer 32 until a predetermined number of samples arecollected. In one embodiment, the apparatus 10 of the present inventionwaits until there are 16 samples in the FIFO buffer 32 before the datafrom the FIFO buffer 32 are read out and are supplied to the DSPmicroprocessor bus 36 and are then further processed.

Further, the eight most significant bits (MSB) of the ten bits from theanalog-to-digital converter 28 of each sensor sample are stored in an8-bit latch 34. The output of the 8-bit latch 34 is supplied to thecontrol microprocessor bus 52 and can be supplied to an external orinternal display screen 74. By outputting the relative signal strengthof the sensor assembly 12 and displaying it on the display 74, theapparatus 10 permits the operator to move the sensor assembly 12 tooptimally place the sensor assembly 12 on the patient.

After the FIFO buffer 32 is filled with the predetermined number ofdigitized sensor samples, the digital signal processor (DSP) 38 isinterrupted. The contents of the FIFO buffer 32 is then transferred tothe DSP microprocessor bus 36 and into the DSP working memory 42. In oneembodiment, the DSP working memory 42 comprises 16K by 16 bits of randomaccess memory (RAM) 46 and two multiplexed memory address counters 44.One memory address counter addresses RAM 46 when writing data to the RAM46. The other memory address counter addresses RAM 46 when reading datafrom RAM 46. Each counter can be set to automatically increment ordecrement after a read or write operation is completed. This increasesprocessing throughput, since the DSP microprocessor 38 does not have toaddress each operation. The memory address counters 44 supply 14 addresslines to address the 16K by 16 RAM memory 46. The DSP microprocessor 38is under the control of a program which is stored in a program memory40. The DSP microprocessor 38 is a TMS-32010 made by Texas Instruments.The program memory 40 comprises 4K by 16 bits of PROM.

The program stored in the program memory 40 controls the DSP processor38 to process the data from the FIFO buffer 32 to convert the sensordata into data values of a signal representative of the output bloodpressure. A copy of the program is attached herewith as Exhibit A. Thefunction of the program stored in the program memory 40 will bedescribed hereinafter.

After the DSP microprocessor 38 has completed the processing of thesensor signal data from the FIFO buffer 32 and into a waveform signaldata, the waveform signal data are passed to and stored in the monitordata FIFO buffer 50.

From the monitor data FIFO buffer 50, the computed points of thewaveform signal may be directed to display on the display screen 74. Thehandshake register 48 links the DSP microprocessor 38 with the controlmicroprocessor 54, by means of the DSP bus 36 and the control bus 52.Both data and control commands can be passed between the two processorsby this register 48. Since the monitor data FIFO buffer 50 is loadedwith and relieved of data at the same rate as the original sensor dataFIFO buffer 32, the computed points which comprise the waveform aredisplayed at the rate of one data point for each four milliseconds.

The control microprocessor 54 also operates under a program which isstored in the program memory 60. A copy of that program is attachedherewith as Exhibit B. The control microprocessor 54 is a 80C31 made byIntel Corporation.

The apparatus 10 of the present invention can also supply the waveformsignal data from the DSP microprocessor 38 to an external patientmonitor 68. When the apparatus 10 functions in this manner, it is actingas an arterial sensor emulator, i.e. to the patient monitor 68, thesignal supplied thereto is no different than the signal generated by anarterial sensor. The data is supplied to an eight-bit multiplyingdigital-to-analog converter 64 which emulates a normal dome pressuretransducer, which is conventionally used to convert invasive arterialline fluid pressure sensor to electrical signals for input to thepatient monitor 68. The monitor 68 outputs its normal excitation voltageand, in turn, receives an input signal of the expected sensitivityexpressed as microvolts output per volt of excitation per millimeter ofmercury of pressure from the analog simulator 64.

The analog simulator 64 consists of an 8-bit multiplyingdigital-to-analog converter 66, which converts digital values receivedfrom the monitor data FIFO buffer 50 sent along the control bus 52, intoan analog signal, required by the external patient monitor 68. Theprogrammable amplifier/low pass filter 65 varies the output level of theanalog signal in accordance with the input sensitivity of the particularpatient monitor 68 being used. Thus, some of the commonly used patientmonitors 68 require the following input sensitivities:

Hewlett-Packard: 5 uv input/volt of excitation/mm Hg.

Marquette: 20 uv input/volt of excitation/mm Hg.

In the operation of the apparatus 10 of the present invention, theapparatus 10 is initially placed in a calibration mode. In thiscalibration mode, the operator identifies the patient monitor 68 withwhich the apparatus 10 is connected through the analog simulator 64. Theselection of the particular patient monitor 68 sets the programmableamplifier 65 to the correct output voltages.

Thereafter, the apparatus 10 sends to the patient monitor 68 a signalthat the patient monitor 68 expects during the venting of the arterialline dome that is normally connected to the arterial line port. When theoperator confirms the completion of the above steps, the apparatus 10then shifts into the next mode of operation, the acquire mode.

In the acquire mode, the apparatus 10 provides a check that the sensorassembly 12 is connected to the apparatus 10. As previously discussed,this includes supplying the excitation signal and the detection of theresponse signal. The detection of the response signal from the sensingfilm 14 indicates that the sensor assembly 12 is electrically connectedand the sensing film 14 is electromechanically operational. Theapparatus 10 acquires and displays the raw pulse vibration signal thatthe sensor assembly 12 detects for the purpose of positioning the sensorassembly 12 optimally. Upon command by the operator, the apparatus 10acquires and stores the previous four-second segment of the pulsevibration signal for correlation with the starting blood pressurevalues.

The DSP microprocessor 38 then processes the signal corresponding to thefour-second segment from the FIFO buffer 32. The DSP microprocessor 38determines the characteristic components of the input signal inaccordance with the following steps:

1. The input signal comprising approximately four (4) seconds of data ata sample frequency of 4 milliseconds is stored in the working memory 42(FIG. 3a).

2. The input signal is fast fourier transformed at approximately 0.25Hz. interval from approximately 0-250 Hz., thereby generating 1024frequency values, with each frequency value having an amplitude value(FIG. 3d).

3. The frequency that corresponds to the maximum amplitude value isdetermined.

4. All the amplitudes of the spectrum data are normalized to thefrequency found in step 3, with that amplitude as 100.

5. The normalized fast fourier transformed signal is stored in theworking memory 42.

After the DSP microprocessor 38 has completed the processing of thesensor signal from the FIFO buffer 32 the apparatus 10 shifts to the BPinput mode.

In the BP input mode, the apparatus 10 requests the operator to inputthe patient's absolute blood pressure values as measured by anindependent blood pressure apparatus. The absolute blood pressure valuesmay be manually determined by a health care worker and entered into theapparatus 10 via the panel buttons 78. Alternatively, the absolute bloodpressure values may be supplied from a conventional, well known,automatic blood pressure monitor 90, such as a pressure cuff measuringinstrument 90. The data from the blood pressure monitor 90 is suppliedto the apparatus 10 along an RS-232 input line 92, to the LCD controller70, just like the inputs from the panel buttons 78. Since the apparatus10 of the present invention 10 requires correlation with absolute bloodpressure value only during the initial stage, the automatic bloodpressure monitor 90, even if it were of pressure cuff design, would notpose the disadvantages of continual use.

The absolute blood pressure values correspond to the correlated, storedfour-second segment sensed by the sensor assembly 12. When the operatorconfirms the conclusion of the input blood pressure value, the DSPmicroprocessor 38 performs the following steps:

a. The four-seconds of data stored in memory 42 (from step 1, above) areintegrated (FIG. 3b).

b. A first scale factor is calculated. The first scale factor is chosensuch that the maximum absolute amplitude of the waveform signal found instep a times the first scale factor would equal the pulse pressure(pulse pressure=systolic--diastolic) (FIG. 3c).

c. An offset (equal to the diastolic) is added to the waveform of step b(FIG. 3c).

d. The processed waveform signal from step c is stored in the memory 42.

In sum, during the input mode, the DSP microprocessor 38 calculates thefirst scale factor and the offset. The waveform generated by theapparatus 10 (from step c), after passing through the analog simulator64, would be as if the waveform signal were generated by an invasivearterial sensor. Thus, the apparatus 10 emulates an invasive arterialsensor. The apparatus 10 then enters into the monitor mode.

In the monitoring mode, the apparatus 10 continuously receives thesignal from the output of the sensor assembly 12 and continuouslyprocesses that signal via the input amplifier 20, digital-to-analogconverter 28 and the sensor data FIFO buffer 32. The DSP microprocessor38 continuously processes the digitized signal in the following manner.

6. The digitized signal from the FIFO buffer 32 is processed inaccordance with steps 1-5 above.

7. The normalized frequency spectrum of the current signal from step 6is subtracted from the normalized frequency spectrum of the initialsignal, as stored in working memory 42 (FIG. 3e or 3f).

8. A comparison is made between the maximum frequency of the normalizedfrequency of the current signal and of the initial signal.

9. The change in the amount of the maximum frequency is noted and issupplied to the following look up table, and the corresponding secondscale factor is determined:

    ______________________________________                                        Change of Maximum Frequency                                                                       Second Scale Factor                                       ______________________________________                                         50%                 85%                                                       67%                 90%                                                       83%                 95%                                                      100%                100%                                                      133%                105%                                                      167%                110%                                                      200%                115%                                                      ______________________________________                                    

10. A comparison is made between the frequency of the maximum amplitudeof the current signal to the initial signal. For each shift in frequency(+ or -) of 1 Hz., the offset (determined from step c) is adjustedcorrespondingly to yield a change of 5 mm Hg. Thus, if the frequency ofthe maximum amplitude of the current signal is increased 1 Hz. from thefrequency of the maximum amplitude of the initial signal, the offset forthe initial signal is increased by an amount equal to 5 mm Hg.

11. The data points from the FIFO buffer 32 are integrated.

12. The data points from step 11 are multiplied by the first scalefactor and by the second scale factor, and the new offset (from step 10)is added to the resultant waveform signal.

The points that represent the waveform signal (step 12) are supplied tothe monitor data FIFO buffer 50 and then to the analog simulator 64 orLCD controller 70 for display on the external patient monitor 68 orinternal LCD display 74. The steps of 6-12 are repeated for each newfour-second segment of data.

In another embodiment of the present invention, the health care workercan supply operating limits to the apparatus 10 by way of the inputpanel buttons 78. When the current signal, processed by the DSPmicroprocessor 38, is outside of the operating limits, an alarm can besounded to alert the health care worker. Alternatively, the LCDcontroller 70 can initiate a control signal to the automatic bloodpressure monitor 90 to re-initiate the apparatus 10.

From the foregoing, it can be seen that a highly accurate and efficientblood pressure measuring apparatus and method has been disclosed.

What is claimed is:
 1. A medical instrument comprising:anelectromechanical sensing means for generating a signal in response tomechanical motion detected by said sensing means, and for moving inresponse to an electrical signal supplied thereto; means for supplyingan excitation signal to said sensing means to cause motion in saidsensing means; and means for detecting a response signal generated bysaid sensing means moving in response to said excitation signal suppliedthereto, wherein said detection of said response signal is indicative ofthe operational readiness of said sensing means.
 2. A method of testingthe operational readiness of an electromechanical sensing means forplacement on a particular location of a patient, said sensing meansgenerating a response signal in response to the detection of mechanicalmotion of said sensing means, said sensing means also moving in responseto an excitation signal applied thereto; said method comprising thesteps of:positioning said sensing means on said patient; supplying anexcitation signal to said sensing means; moving said sensing means inresponse to said excitation signal; and detecting said response signalin response to the movement of said sensing means, wherein the detectionof said response signal is indicative of the operational readiness ofthe sensing means.